Habitat selection : how sensory systems influence settlement patterns in larval red drum (Sciaenops ocellatus)

Settlement is arguably the last stage of high mortality in the life cycle of many marine fish species with a bipartite life cycle, making the number of larvae that settle successfully to a benthic habitat a reasonable determinant of future population size. Habitat selection during settlement is likely an active process, however, much of what we know about settlement behavior comes from studies in coral reef ecosystems. This collection of studies examined the ability of estuarine-dependent fish larvae (red drum, Sciaenops ocellatus) to locate settlement sites based on information received from their senses, with a focus on the different spatial scales over which larval sensory systems operate. During the pelagic phase, red drum are exposed to elevated sound levels in the tidal inlets. This noise caused larvae to reduce their activity in the form of fewer turns, less time spent swimming, and a lower overall mean speed compared with silent controls. As red drum approached settlement size, but not at earlier stages, they responded to olfactory cues associated with seagrass beds, their primary settlement habitat. Activity increased in the presence of lignin, a compound associated with the cell walls of vascular plants. Also, settlement-size larvae spent more time in water masses taken from seagrass beds compared to control sea water, implying a preference for olfactory cues associated with estuaries. While settlement and post-settlement-size larvae positioned themselves near the benthos regardless of water column height or substrate color, pre-settlement larvae moved away from yellow and green (associated with the estuary) but not black (associated with a deep water column) benthic colors. Additionally, red drum larvae settled to seagrass and sand at a smaller size than they did to oyster shells, and they delayed settlement when a suitable benthic habitat was not available. These findings can be interpreted as evidence for an estuarine-dependent species taking advantage of cues available to multiple sensory systems in order to actively locate settlement habitats.